The invention relates to a light-emitting device provided with at least one light-emitting diode and a phosphor layer.
Solid-state light sources, for example light-emitting diodes, in particular semiconductor diodes, have been known for years. The light emission of semiconductor diodes is based on the recombination of electron-hole pairs (excitons) in the transition region of a pn-transition in a semiconductor which is forward-biased. The size of the bandgap of the semiconductor roughly determines the wavelength of the emitted light.
Semiconductor diodes which radiate visible light are also used in color displays. In these color displays, the three constituent colors red, green, and blue are generated by arrays of red-, green-, and blue-emitting semiconductor diodes. A problem here is, however, how to achieve a true-to-life color picture rendering, in particular a true rendering of green and blue in the pictures.
The development of semiconductor diodes which emit UV radiation has increased the possibilities for a true color picture representation on color picture screens with diode addressing. The combination of semiconductor diodes which emit UV radiation with phosphors which convert UV radiation into visible light renders it possible to represent any color of the visible light which may be desired by means of a semiconductor diode, as well as white. Such a color display is known, for example, from DE 19800983 A1. This principle is also applicable to semiconductor diodes which emit violet or blue light, provided suitable phosphors are used.
A major advantage of light-emitting semiconductor diodes in comparison with traditional lamps is their high stability and accordingly their long useful life. A limiting factor for color picture screens with diode addressing may be the stability of the phosphors used in the phosphor layer. Since the phosphors are not fully isolated from the surrounding atmosphere, water-sensitive phosphors may become hydrolyzed by moisture from the air. Comparatively stable phosphors may also be hydrolyzed under the influence of high temperatures and humidity. The degradation of the phosphors in the phosphor layer shortens the useful life of a light-emitting device.
It is an object of the present invention to provide a light-emitting device which is provided with a light-emitting diode and a phosphor layer and which has an improved useful life.
This object is achieved by means of a light-emitting device provided with at least one light-emitting diode and a phosphor layer which comprises a phosphor with a coating.
A degradation caused by moisture from the air is prevented by the coating of the phosphor particle with a dense, water-resistant film.
Preferably, the coating is chosen from the group comprising organic materials, inorganic materials, and glass materials.
It may be preferred that the organic material is chosen from the group comprising latex and polyorganosiloxane.
It may furthermore be preferred that the glass material is chosen from the group comprising borosilicates, phosphosilicates, and alkali silicates.
It may furthermore be preferred that the inorganic material is chosen from the group comprising oxides, borates, phosphates, and combinations of these materials.
These organic, glass-type, or inorganic materials form thin, water-insoluble coatings on the phosphor particles, they do not react with the phosphors, and they are not degraded by UV radiation or radiation with a wavelength between 410 and 450 nm. Furthermore, they are colorless and thus do not influence the color values of the phosphors.
An advantageous embodiment is characterized in that the phosphate is an orthophosphate MPO4, in which M is chosen from the group comprising Al, La, Sc, Y, and Lu, or a polyphosphate with a chain length n of between 101 and 106 and having a composition (M0.5PO3)n, in which M is chosen from the group of Ca, Sr, and Ba.
These phosphates form particularly satisfactory, closed films with a dense surface on the phosphors.
It is preferred that the phosphor is chosen from the group comprising oxidic phosphors, sulfidic phosphors, aluminate phosphors, borate phosphors, vanadate phosphors, and silicate phosphors.
These phosphors in combination with suitable activators effectively convert UV radiation or blue light into visible light of greater wavelength.
It is preferred that the aluminate phosphor is chosen from the group comprising Y3Al5O12:Ce, (Y,Gd)3(Al,Ga)5O12:Ce and BaMgAl10O17:Eu,Mn.